The present pertains to a Correlating Shack-Hartmann wavefront sensor High resolution observations of the Sun have become increasingly important for solving many of the outstanding problems in solar physics. Near diffraction limited snapshot images taken for example at the 76 cm Vacuum Tower Telescope (VTT) at Sacramento Peak show an amazing amount of fine structure. In these images, small magnetic flux elements are visible as bright points. Images of this quality are rare. More importantly, in order to study the physics of these flux elements, or solar fine structure in general, spectroscopy, and polarimetry of the fine structure is required. The exposure times are then typically of order 1 second and the resolution currently achieved in spectroscopic/polarimetric data on a more or less regular basis is typically 0.xe2x80x35-1xe2x80x3.0, which is insufficient to effectively study solar fine structure. NSO is in the process of implementing adaptive optics at the VTT, which will allow diffraction limit spectroscopy and increase the scientific productivity of this high resolution facility.
Further motivation for the development of solar adaptive optics is provided by theoretical model predictions. Sophisticated simulations of magneto hydrodynamic processes predict structure below the resolution limit of current solar telescopes, which have apertures of order 1 meter. These theoretical predictions are reinforced by polarimetric measurements, which provide a filling factor (the ratio of magnetic to non-magnetic area) and thus give an indirect measurement of the size of the magnetic elements present in the solar surface. Observations of solar structures below the resolution limit of about 0.xe2x80x32 of current solar telescopes are needed to study the important physical processes that occur on such small scales. A new generation solar telescope with a substantially increased aperture is needed. Solar telescope designs with apertures between 2.5 and 4 mm are being discussed. However, adaptive optics is essential to achieve consistent spatial resolution of xe2x89xa60.xe2x80x31 from ground based observatories.
The Hartmann test was developed a century ago as a method for aligning large optical systems. About 30 years ago, Shack had the idea of replacing the perforated screen with a lenslet array, thereby allowing the entire system pupil to be sampled. By adding quadrant detectors or a fast readout CCD to detect the lenslet images, the ordinary Shack-Hartmann wavefront sensor was developed.
This WFS is now widely used, but its main limitation is that it is restricted to point sources as sensing targets. This became a principle driver for the development of high power laser beacons at SOR and elsewhere.
The present invention is a Correlating Shack-Hartmann wavefront sensor developed for the solar adaptive optics systems. We developed the correlation tracker as an image stabilization technique. This tracker compares a real time image with a stored reference image using a fast cross-correlation calculation. Trackers of this sort are gradually coming into widespread use at solar observatories. One of their most useful features is their ability to track an extended, low contrast, temporally evolving scene. The development of adaptive optics for solar imaging has always been held back by the lack of a wavefront sensor capable of functioning with an extended scene as its target. The solar disk is too bright for laser beacons. We realized some years ago that by replacing each simple quadcell or equivalent detector in the standard Shack-Hartmann WFS with a correlation tracker, we could build an extended source WFSxe2x80x94what we now call the xe2x80x9cCorrelating Shack-Hartmannxe2x80x9d WFS. The main technical hurdles were obtaining a large format, fast readout CCD.
The task of providing a Correlating Shack-Hartmann WFS system is alleviated, to some extent, by the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 5,912,731, Jun. 15, 1999, Hartmann-type optical wavefront sensor, DeLong, Raymond K.,6
U.S. Pat. No. 5,610,707, Mar. 11, 1997, Wavefront sensor for a staring imager, Duncan, Alan
U.S. Pat. No. 5,598,261, Jan. 28, 1997, Wavefront sensor for a scanning imager, Duncan, Alan L
U.S. Pat. No. 5,300,766, Apr. 5, 1994, Scanning scene-based wavefront sensor having a linear image sensor array and a pupil sensor array, Granger, Edward M.
U.S. Pat. No. 5,146,073, Sep. 8, 1992, Linear wavefront sensor camera with deformable mirror for recording velocity compensated images et al.
The above-cited patents disclose scanning scene-based wavefront sensors. A need remains to provide scanning scene-based wavefront sensor system that can be used as a solar imager. The present invention is intended to satisfy that need.
The present invention is a Correlating Shack-Hartmann wavefront sensor developed for the solar adaptive optics systems. We developed the correlation tracker as an image stabilization technique. This tracker compares a real time image with a stored reference image using a fast cross-correlation calculation. Trackers of this sort are gradually coming into widespread use at solar observatories. One of their most useful features is their ability to track an extended, low contrast, temporally evolving scene.
It is an object of the invention to provide a Correlating Shack-Hartmann wavefront sensor developed for an image stabilization tracker that compares a real time image with a stored reference image using a fast cross-correlation calculation.
These objects together with other objects, features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings wherein like elements are given like reference numerals throughout.